Asynchronous generators of the state of the art typically comprise a rotor and a stator with stator coils. Oftentimes, the stator core is built using a laminate of thin metal sheets.
In order to have the asynchronous generator to operate properly, the stator coils are supplied with an alternating voltage. This alternating voltage induces an alternating magnetic flux which, in systems of the state of the art, causes the stator core and the stator coils to vibrate.
Such vibration, however, produces a sound having the frequency of the alternating voltage supplied to the stator coils.
Particularly when used with wind turbines, the sound thus produced by the asynchronous generators is often found irritating by people in the immediate vicinity of the wind turbine, for instance by people living next to a wind turbine or a wind turbine park.
In order to reduce the noise thus generated by wind turbines, systems of the state of the art use noise reducing cladding for the generator housing of the wind turbine, which leads to increased material cost and additional technical effort in the manufacturing of wind turbines.
US 2003/052643 A1 discloses a brushless doubly-fed induction machine control in which a first stator and a second stator are present. In the first stator, the output power increases lineally with rotor speed. In the second stator, the output power is not dependent on the rotor speed and constant over the whole speed range. Normal full machine excitation is applied by a current controller.
S. Müller et al. “Adjustable speed generators for wind turbines based on doubly-fed induction machines and 4-quadrant IGBT converters linked to the rotor”, Industry Applications Conference, 2000, Conference Record of the 2000 IEEE 8-12 Oct. 2000, Piscataway, N.J., USA, IEEE, vol. 4, 8 Oct. 2000, pages 2249-2254 describe an investigation of a doubly-fed induction generator system as an alternative to adjust speed over a wide range.
E. Levi et al. “Study of main flux saturation effects in field-oriented induction motor drives”, 19891106, 19891106-19891110, 6 Nov. 1989, pages 219-224 describe a study of main flux saturation effect in field-oriented induction motor drives.